1. The Field of the Invention
The invention generally relates to cost-effective methods for building and operating metro area networks. More specifically, the invention relates to scaling an existing coarse wavelength division multiplexing infrastructure by using dense wavelength division multiplexing signals.
2. Description of the Related Art
In the field of data transmission, one method of efficiently transporting data is through the use of fiber optics. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Light signals are also resistant to electromagnetic interference that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
It is often desirable in the networking context to maximize the amount of data that can be propagated through the network. It is also desirable for economic reasons to minimize the hardware used to create the network infrastructure. To accomplish both of these objectives, multiplexing schemes are used to transmit multiple signals along a single data path such as a fiber optic fiber. One particularly useful and successful method of multiplexing is wavelength division multiplexing. In the fiber-optic systems, wavelength division multiplexing includes transmitting various individual signals along a single fiber, with each signal being used to transmit a different light wavelength. To accomplish wavelength division multiplexing, several specialized optical components are needed, including demultiplexers (demuxes), multiplexers (muxes), mux/demux modules, and optical add drop multiplexers (OADMs).
A demultiplexer generally takes as its input an optical transmission that includes a number of individual signals, with each signal being transmitted using a particular wavelength of light. An exemplary optical demultiplexer is shown in FIG. 1 and designated generally as 100. The optical demultiplexer 100 has an input port 102. The input port 102 receives a multiplexed transmission 104. In the present example, the multiplexed transmission 104 has four individual signals, each of different wavelengths, which are designated in this example as λ1, λ2, λ3, and λ4, as indicated in FIG. 1A. The optical demultiplexer 100 is a passive device, meaning that no external power or control is needed to operate the device. Although, in this example, the optical demultiplexer 100 is a passive device, it should be noted that active devices can be used in optical demultiplexing as well. Using a combination of passive components, such as thin-film three-port devices, mirrors, birefringent crystals, etc., the optical demultiplexer 100 separates the multiplexed signal 104 into its constituent parts. Each of the individual wavelengths, each representing a separate signal on a communication channel, is then output to one of output ports 106, 108, 110, 112.
A multiplexer functions in the inverse manner as the demultiplexer. Multiplexers can often be constructed from demultiplexers simply by using the output ports 106, 108, 110, 112 as input ports and the input port 102 as an output port.
An optical device that combines the functionality of a demultiplexer and a multiplexer is known as a mux/demux. An exemplary mux/demux is shown in FIG. 2 and designated generally as 200. The mux/demux 200 has a multiplexed input port 202 that accepts as its input a multiplexed transmission 104. The multiplexed transmission 104 is separated into its constituent parts and output to demultiplexed output ports 204, 206, 208, 210. In a multiplexing operation, demultiplexed input ports 212, 214, 216, 218 accept as their input individual signals, with each signal being encoded on a different optical wavelength. The individual signals are combined into a multiplexed transmission and output to the multiplexed output 220 from output port 105.
An OADM is a component designed to extract an individual signal from the multiplexed transmission while allowing the remaining signals on the multiplexed transmission to pass through. The OADM also has an add port that can be used to remix the extracted signal with the multiplexed transmission or to transmit other data onto the fiber-optic network. An example of an OADM is shown in FIG. 3 and designated generally as 300. The OADM 300 is designed for bi-directional data communication. In optical networks, to distinguish the direction of data travel, the directions are referred to as east and west directions. In FIG. 3, data that travels in an easterly direction travels to the right of the OADM 300. Data the travels in a westerly direction travels to the left of the OADM 300.
Now illustrating the functionality of the OADM 300, a multiplexed transmission 104 is input into the east input port 302. The OADM 300 is designed for a specific wavelength or, more precisely, a band of wavelengths. For example, if the particular multiplexed transmission has four wavelengths, including a 1510 nanometer wavelength, a 1530 nanometer wavelength, a 1550 nanometer wavelength, and a 1570 nanometer wavelength, and the OADM 300 is designed to extract signals transmitted on the 1550 nanometer wavelength, the OADM may in fact extract any signal within a 12 nanometer bandwidth centered about the 1550 nanometer wavelength. As such, any wavelength between 1544 and 1556 nm is extracted by the OADM 300. In the present example, an individual signal 304 is extracted from the multiplexed transmission 104 and output to a device existing on the network, such as a network node 306, through the east drop port 308.
All other wavelengths remaining on the multiplexed transmission 104 continue through the OADM 300 and exit through an east output port 310, where they may continue to propagate on the fiber-optic network. If the OADM is a bi-directional module, such as OADM 300, a multiplexed transmission traveling in a westerly direction enters the OADM 300 at the west input port 318, drops the particular signal through the west drop port 320, adds a signal through the west add port 322, and propagates the remaining wavelengths through the west output port 324.
The network node 306 has two transceiver modules 312. In one embodiment, the transceiver modules may be GigaBit Interface Converters (GBICs). The transceiver modules 312 have an input port for accepting optical signals so that the signals can be converted to a data signal useful by the network node 306, and output ports for generating optical signals from the network node 306 so that data from the network node 306 may be propagated on the fiber-optic network. Optical signals from the network node 306 may be propagated onto the fiber-optic network such that they travel in an easterly direction by inputting the signals into the east add port 326 or propagated to the fiber-optic network, such that they travel in an westerly direction by inputting the signal signals into the west add port 322. By using an OADM that is bi-directional, redundancy may be added to the optical fiber network to provide for such contingencies as broken fibers in one of the directions. Optical add drop modules, such as OADM 300, are generally passive devices and are constructed using thin-film three-port devices, fused fiber devices, or other passive components.
One especially useful implementation of wavelength division multiplexing manifests itself in metro area network design. Applications of metro area networks may include remote storage services, intra-enterprise communication and high-speed data services. When designing metro area networks, designers face a particular set of challenges. For example, designers typically try to design metro area networks to have enough capacity that potential customers will find it worthwhile to purchase services from the metro area network provider. On the other hand, in the highly competitive arena of network services, network designers do not want to design an expensive network with high available bandwidth without being able to attract a sufficient number of customers to make the high bandwidth network profitable. Accordingly, there exists a need for a solution for designers by which more cost-efficient low bandwidth components can be used while preserving the option of efficiently upgrading the metro area network to higher bandwidth components.